Using native dependencies inside Maven - maven-2

Question

A POM dependency contains native libraries (DLLs inside a JAR file). How do I programmatically look up the path of the downloaded JAR file so I can pass it into "java.library.path"?

Answer 1

Answering my own question: http://web.archive.org/web/20120308042202/http://www.buildanddeploy.com/node/17
In short, you can use the maven-dependency-plugin:unpack goal to extract the libraries into a known path, and pass that into java.library.path:
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-dependency-plugin</artifactId>
<executions>
<execution>
<id>unpack</id>
<phase>compile</phase>
<goals>
<goal>unpack</goal>
</goals>
<configuration>
<artifactItems>
<artifactItem>
<groupId>org.jdesktop</groupId>
<artifactId>jdic-native</artifactId>
<version>${jdic.version}</version>
<classifier>${build.type}</classifier>
<type>jar</type>
<overWrite>true</overWrite>
<outputDirectory>${project.build.directory}/lib</outputDirectory>
</artifactItem>
</artifactItems>
</configuration>
</execution>
</executions>
</plugin>

Answer 2

Since System.load() can't load libraries from within a jar, you will have to use a custom loader which extracts the library to a temporary file at runtime. Projects With JNI discusses this approach and provide code for the custom loader.
Library loader
We now have our JNI library on the
class path, so we need a way of
loading it. I created a separate
project which would extract JNI
libraries from the class path, then
load them. Find it at
http://opensource.mxtelecom.com/maven/repo/com/wapmx/native/mx-native-loader/1.2/.
This is added as a dependency to the
pom, obviously.
To use it, call
com.wapmx.nativeutils.jniloader.NativeLoader.loadLibrary(libname).
More information is in the javadoc for
NativeLoader.
I generally prefer to wrap such things
in a try/catch block, as follows:
public class Sqrt {
static {
try {
NativeLoader.loadLibrary("sqrt");
} catch (Throwable e) {
e.printStackTrace();
System.exit(1);
}
}
/* ... class body ... */
}
An alternative would be to unpack the dependency, for example using dependency:unpack.

Answer 3

You can use the maven dependency plugin to copy the artifacts to a predefined path:
http://maven.apache.org/plugins/maven-dependency-plugin/examples/copying-artifacts.html

Answer 4

If the DLL is inside the JAR, then you will need to copy it out to a directory before it can be loaded. (JARs that include native libraries usually do this themselves.) If your JAR isn't doing this, then you can use Class.getResourceAsStream() and write this to a directory that you've added to the java.library.path.
For an example of this, see loadNativeLibrary in JNA. It uses this technique to load it's own library (a JNI library) from a JAR.